This invention relates to strain sensors of a resistance element utilizing the phenomena of a resistance element which changes its electrical resistance in response to a variation in strain.
In recent years, strain sensors have been widely used to detect the magnitude of stress and load as applied to the various places of machines, ships, automobiles, etc. One of the representative types of the strain sensors is one which has a structure with thin film resistance elements made of Cu-Ni alloy, Ni-Cr alloy, or similar material, formed by deposition or sputtering on resin films of polyester, epoxy, polyimide, etc. Also, as disclosed in Japanese Patent Publication No 03-20682/91, there is a strain sensor which uses glass plates in place of the resin films as described above.
These strain sensors are usually attached by applying an adhesive cyano-acrylate resin onto the surface of the places where stress or load is to be measured. In the case of glass plate sensors, the glass plate is attached by melting to the measurement surface.
When an external stress or load is applied to a body, the strain caused by the external stress or load is propagated through the resin film or the glass plate to the resistance element. Such a propagated strain changes the cross-sectional area of the resistance element causing a variation in electrical resistance values of the resistance element. By detecting the variation in resistance in the form of an electrical signal, the magnitude of strain is measured and consequently the stress or load as imposed onto the measurement place is known.
Incidentally, one of the largest applications of strain sensors is the suspension mechanism for vehicles such as automobiles, etc. In this application, strain sensors are attached by applying adhesive resin to the surface of shafts, for example, of suspension mechanisms. The strain sensors as installed in this way will detect the load of the vehicle body as applied through the wheels.
However, in the application of vehicle suspensions, the operating temperature ranges from -50.degree. C. to 150.degree. C. and the maximum load reaches as much as 2 tons. When the strain sensors are used under such a severe environment for a long period, the strength of the adhesive resin is deteriorated and the sensors peel off from the surface of the measurement places. Especially, in the case of glass plate sensors, the glass plates have to be attached by melting onto the curved surface of measurement bodies like wheel shafts. Coupled with the problem of rather weak adhesiveness of glass plates, the strain sensors of the glass plate structures are liable to peel off easily. Therefore, a strain sensor of high quality and excellent durability is desired so that the sensor does not peel off after a long period of usage under such severe environmental conditions as described above.